Djokovic, 38, beat Tien, 6-1, 7-6, 6-2 to improve to 19-0 in US Open first-round matches, tying Roger Federer for first all-time. But he labored with low energy, a nasty blister on his right big toe and Tien’s level, especially in the second set.

As a tennis veteran, Djokovic will know match sharpness is very different to playing practice sets, so it was no surprise that his physical levels collapsed mid-way through his match against the young and hungry Tien.

Grand Slam champion Novak Djokovic secured his 75th consecutive first-round win at a Grand Slam as he defeated American Learner Tien in straight sets in the first-ever Sunday night session match at the Arthur Ashe Stadium.